Gold recovery



March 4, 1969 L. E. LANCY GOLD RECOVERY I of 3 Sheet Filed Nov. 22, 1965my 2 Na WL m5 m l a L fi/W Y B ll o Wm mmmm M? I nwmmn l|.|l ||l lilllllon -ll| in m f L. E. LANCY GOL-D RECOVERY March 4, 1969 Sheet Filed Nov.22, 1965 INVENTOR. Les/1e .E Laney H/S ATTORNEYS United States Patent3,431,187 GOLD RECOVERY Ellwood City, Pa., assignor to Laney Inc.,Zelienople, Pa., a corporation of This invention relates to the recoveryof gold carried over from a plating operation and particularly, to newand improved procedure and apparatus for removing gold from dragout orwashing solutions.

The commercial plating of gold metal has many fields of use and isparticularly important in the electronics field where lines are used forplating electrical contacts, printed circuits, etc. Each production linecustomarily has its own electrolytic gold plating solution. Due to thescarcity of gold and its cost, it is important to effect a maximumutilization of the metal in operations of this type. Gold from thesolution, of course, is plated out on the surfaces which are undergoingthe processing, but it has been found that a considerable amount is lostin the form of carry-over or dragout from the plating solution. That is,wet film droplets adhere to the surfaces of the work pieces when theyare moved out of or removed from the gold plating solution.

There is a need for a recovery operation that will be practical foreconomically recovering gold dragout from various types of platingsolutions which, in the new in stallations, involve electroplatingsolutions in the neutral or in the acid pH range. Also, some platinglines make use of cyanide solutions. The electroplating baths usuallycontain about 1 /2 to 8 g./l. of gold in solution.

There have been a number of methods employed in endeavoring to recoverwaste gold from such solutions and all to date have made use of means atthe end of each individual plating line for this purpose. I have foundthat there is a need for a recovery system or procedure which will makepossible the combining of the dragout solutions from a number of platinglines in an industrial plant and the recovery of gold in the plant in apractical, efiicient, and effective manner.

In endeavoring to recover gold, it has been the usual industrialpractice to wash the workpiece with a stagnant water bath which iscalled a dragout recovery aqueous solution. It will be apparent that thegold concentration in the dragout solution will progressively increaseand that unless the solution is exchanged for a new water solution, thegold concentration will tend to approach the same concentration asmaintained in the gold plating solution, itself. Depending upon the Workload and in accordance with one method, the dragout solution from theend of each individual plating line is bottled once a month and shippedto the refinery for gold recovery.

Another approach to gold recovery has been to pass each dragout solutionthrough an ion exchange cartridge, so that the ion exchange materialwill pick up the gold salts as the solution is circulated through thecartridge. This resin cartridge is then returned to the refineryoperator who burns the resin to reclaim the gold value. These tworecovery processes are cumbersome and costly and involve considerablemanipulation. Although I have found that gold may be obtained from aplating solution by electrolysis when the concentration in the dragoutsolution approaches about 500 to 600' mg./l., the usual gold deposit ispowdery and non-adherent. Thus, electrolysis for gold recovery has notbeen practiced by those skilled in the art because the electrolyticallyregained gold is lost as a powder in the electrolyte solution. I haveinvestigated the possibility of filtering out this powder, but havefound that with the quantities in question, it is uneconomical on thebasis of the filter areas that have to be maintained and the need toburn the filter paper to recover the small quantity of gold dust.

I have also determined that gold recovery from a neutral or acid goldsolution is even more hampered and that the efliciency of theelectrolytic recovery approaches zero with a gold concentration insolution of about 200 mg/l. or less. Cathode efiiciency is considered tobe the ratio between the actual weight of the deposit gained against thecalculated theroretical efiiciency from the standpoint of an electricalunit employed for metal deposition. The efiiciency of the cathode usedfor electrolytic recovery becomes less and less as the concentration ofthe metal salts decreases. I have also found that acid gold solutionsare particularly adverse to such a recovery operation, in that the goldions are in a higher valency state (Auric III valent). This, of course,means that for the same weight of gold recovery, three times the amountof current is needed than when, for example, valency ions (AurousIvalent) are available.

I have analyzed a number of gold dragout solutions and have found thatthe usual concentration is around or about 1 g./l. and that it is quiteusual to have a dragout of 2 to 4 g./hour. It is now also usual tomaintain three, four or more parallel plating lines, particularly in theelec' tronics field.

In view of the above considerations, it has been an object of myinventionto develop new and improved recovery procedure and means for,in a practical economical manner, saving or recovering the gold valuesthat are carried-over or dragged out from a gold plating solution orline;

Another object of my invention has: been to provide means that maybeeffectively, etficiently and economically used for directly recoveringgold values from the lines at a gold plating installation or plant andwithout the need for shipping the gOld containing dragout solution orother means to a refinery to recover the gold;

A further object of my invention has been to develop practical means foruniversal application to the recovery of gold values from a gold platingline and particularly, from a group or plurality of linessimultaneously, and irrespective of the type of plating solutions usedin the lines;

These and other objects of by invention will appear to those skilled inthe art from the illustrated embodiment and the detailed descriptionthereof.

In the drawings:

FIGURE 1 is a typical plant layout illustrating a system constructed andemployed in accordance with my invention for recovering gold values froma plurality or group of plating lines which lines may have ditferenttypes of plant solutions;

FIGURES 2 to 6 illustrate details of a construction of a suitableelectrolytic recovery tank employing my invention;

FIGURE 2 is a side the tank construction;

FIGURE 3 is a horizontal section of the same scale taken through thetank with its electrodes removed to better illustrate compartmentseparators;

FIGURE 4 is a vertical section on the scale of FIG URES 1 and 2 showingelectrodes in place within compartments;

FIGURE 5 is an enlarged side elevational detail in partial section,taken at an upper corner of the tank showing the mounting of electrodebars; and

FIGURE 6 is an end elevational detail in partial section on the scale ofand taken along the line VI-VI of FIGURE 5.

In devising a solution to the problem heretofore involved in recoveringgold values from dragout solutions,

perspective view in elevation of I have been able to develop a system,procedure or means which will assure that the gold ions are in their lowvalency state, that is effective for recovery of gold from lines usingvarious types of plating solutions, that can be employed at a centralarea or point for recovery of gold from a number of plating lines andirrespective of the particular plating solutions used, and that willprovide a practical and economical means of substantially fullyrecovering the gold content of dragout or washing solutions.

I have developed an alkaline cyanide electrolytic recovery solutionwhich solves the problem and is maintained at a minimum pH that is abovethat at which poisonous cyanide gas will evolve. Such a solutionprovides the gold ions in their low valency state (Aurous I valent) andis non-toxic to operating personnel. For example, if an acid is added toa cyanide solution having a pH or less than 8, hydrocyanic gas isevolved which is highly toxic. In accordance with my invention, I fullyavoid the production of such a gas, and fully effectively andefficiently deposit a gold film on cathodes, even where the dragoutsolution has a gold content of as low as about or 6 milligrams perliter, by maintaining a pH of at least about or higher, with an optimumof about 12. I have been able to maintain the pH, either by employing acontinuous supply of an alkali metal hydroxide, such as caustic soda, toneutralize acid dragin, or through the use of a pH controller and theaddition of the alkali to the solution as based upon the reading of suchcontroller.

I formulate a gold electrolytic recovery solution by the addition of analkali metal cyanide, such as sodium or potassium cyanide, and an alkalimetal hydroxide, such as sodium or potassium hydroxide, to the dragoutor washing water within which the work piece has been rinsed afterleaving one or more plating solutions. The same gold alkaline cyaniderecovery solution may be used, irrespective of the type or types ofsolutions used for providing the desired electrical contact, printedcircuit or other type of workpiece gold plating operation. I recirculatethe treated dragout solution for reuse, so that any minimal gold content(below a concentration of about 5 to 6 mg./liter) is not lost, but maybe removed during the subsequent recirculating or return of the useddragout solution back to the electrolytic recovery tank or cell. In thisway, I also have been able to continuously recover the gold which isentered through dragin from gold plating baths that are in use.

I have been able to maintain a gold recovery electrolyte which iseffective with a gold concentration as low as about 5 or 6 mg./l. and,at the same time, recover all the gold that has entered the system inthe form of an adherent uniform and peelable gold foil, operating at anavailable efiiciency of about 20% and an average current density ofabout 5 amperes per square foot. The electrolytic recovery system makesuse of a plating cell constructed for good mixing, cathode agitation andrecirculation of the recovery solution through the system.

I have found that a minimum of about .5 of an ounce/ gallon of alkalinecyanide (as added, for example, to the bath or tank at station D ofFIGURE 1 of the drawings) is sufiicient for starting the recoveryoperation. As the operation proceeds, the CN level automaticallyincreases, due to the formation of alkaline cyanide in the gold recoverybath (see, for example, the tank or bath 30 at station E of FIGURE 1 ofthe drawings). I prefer an initial addition of cyanide of about 2ounces/gallon, and find that the upper limit is only controlled byeconomic considerations.

Since at about 100% current efficiency, 5.15 amperes per hour would berequired to plate out about 38 grams of gold per hour at an availablecathode efliciency, I have found that about amperes per hour aresufficient to reclaim 38 grams of gold per hour in a plant operating twogold solution lines. In this example, one line was an acid gold platingline containing approximately 7.5 g./l. gold and the other a cyanideplating line containing approximately 25 g./l. of gold in their platingsolution. Assuming one gallon of dragout per hour from each line, thiswould amount to 28.4 g./ hr. from the acid gold solution and about 9.5g./hr. from the cyanide gold solution. By maintaining an electrolyticrecovery system employing 50 to 60 square feet of cathode area, I havebeen able to recover gold dragout at the same rate that it is introducedinto the electrolyte and to maintain an electrolytic recovery solutionhaving a gold content of about 5-20 mg./l. Any loss of gold from such asolution of dragout is of a negligible quantity from the standpoint ofgold recovery, in that 2 gallons of electrolyte solution per hour wouldcontain a total of about mg. of gold in comparison with a recovery ofabout 37.9 grams. The electrode material for the recovery cell or tankmay be of any suitable cathode material, although I have found that astainless steel sheet is highly effective and provides an easy peeledoff gold film of several millimeters in thickness.

In FIGURE 1 I have shown a representative system employing theprinciples of my invention. In this figure, A, B, and C are individualgold plating lines along which work pieces 9, 9 and 9", respectively,are moved or advanced to produce a finished gold plated product orworkpiece at the end of the line, such as indicated by 9a of the line C.By way of example, the line A may employ a cyanide type of gold platingelectrolyte in its plating bath or tank 10, line B may employ a neutraltype of electrolyte plating solution in its plating bath or tank 10, andline C may employ an acid electrolyte plating solution in its platingbath or tank 10". As indicated by the dotted lines and the arrows, theworkpieces of each line enter and pass through the baths or solutions ofthe tanks 10, 10, and 10", at which time, they are provided with asuitable plated surface, and are then moved into aqueous solutions ofdragout recovery tanks 11, 11, and 11 which wash-01f and collect thecarried over residual gold from the workpieces for recovery purposes.Subsequently, the workpieces then leave the recovery tanks as finishedworkpieces, such as represented by 9a of the line C.

It will be noted that washing solution is introduced into each of therecovery tanks 11, 11, and 11" through upstream deli-very pipes 35, 35,and 35", and as controlled by regulating valves 34, 34' and 3 4".Washing solution containing the dragout gold of each of the tanks 11,11', and 11 is taken oif from the tanks from overflow troughs 12, 12,and 12", through downstream delivery branch lines 13, 13, and 13" thatare connected to a main downstream return line 14. The line 14 deliversthe dragout solution from the tanks of each line to a sump tank 15 at asolution conditioning or treating zone D. The sump tank 15 serves tocondition the solution in readiness for the electrolytic removal of itsgold content. A strong neutralizing alkali, such as NaOH, is supplied tothe solution of the tank 15 from a supply tank or vat 16, through line17 and a regulator valve 18, and down pipe 17a. The operation of thevalve 1 8 is controlled by an electric solenoid 19a that iselectrically-connected through a pH controller 19 to an electrode system20 (shown as having two electrode wires). In this manner, the requisiteamount of alkali may be automaticallysupplied or fed to the solution inthe right hand portion of the tank 15 to provide it with the requisitepH, as sensed by the electrode system 20 and controller 19.

The tank 15 is shown provided with a partition member 15a that separatesthe tank into two compartments or zones and provides an overflow edgefor passing treated solution from the right hand compartment or zoneinto the left hand or downstream cmpartment or zone. It will be notedthat a stirrer having an actuating motor 22 (such as an electric motor),a propeller shaft 23 and a propeller or mixing blade 24, is employed toagitate and quickly mix the solution in the right hand compartment.

I have shown an electrode system 21 extending into the solution of theleft hand compartment. It has a short length wire or electrode 21a and alonger length wire or electrode 21b and is electrically-connected to themotor of a pump 26 (see the dotted line of FIGURE 1). When the liquid orsolution level reaches the end of the wire 21a, the pump 26 is actuatedto pump-out treated solution. 0n theother hand, when for any reason, theliquid level falls below the end of the wire 21b, the motor of the pump26 is de-energized to stop the pumping action. Conventional controldevices may be used in this connection, or the electric motor of thepump 26 may be manuallycontrolled on the basis of a visual determinationof the availability of liquid in the left hand zone. In the arrangementshown, the system 21 and pump 26 may be omitted and gravity flowutilized where, for example, tank has an upper positioning with respectto the tank 30' at station E.

The gold containing solution, after being suitably conditioned in thetank 15 to provide an alkaline cyanide solution, is then gravity-flowedor drawn by means of downstream pump 26 from the bottom of the tank 15,through line 27, into a recovery tank or cell 30 at station E. It willbe noted that the solution is properly pre-conditioned at station Dwithin the tank 15 before it is introduced into the container or tank 30at station E. After the gold has been removed from the solution in tank30 by electroplating it out on cathode plates, it is then withdrawnthrough an outflow line 31 by a motor-operated pump 32 through upstreamline 33 which returns the solution to each of the dragout recovery tanks11, 11, and 11", through overhead header supply lines 35, 35', and 35".

In FIGURES 2 to 6, I have illustrated a gold recovering tank orcontainer unit 30 suitable for carrying out my invention. The tank isshown of rectangular shape, having vertically-extending side walls 40,end walls 41, and a closing bottom wall 42 to define an open topcontainer. The tank 30 is reinforced along its outer sides by aframework 44 made up of angle bars or pieces secured in place, asbywelding. Upper portions of the reinforcing framework support atransverse or cross extending group of longitudinally spaced-apartchannel members 45 on which cathode electrode bars 46 and anodeelectrode bars 47 are positioned.

It will be noted from FIGURE 2 that the electrode bars 46 and 47 extendlongitudinally above the open mouth of the container 30 to rest upon thewebs of the support channels 45 which are inverted and which, in turn,rest on the top edges of the tank and the reinforcing framework 44. Asalso shown in FIGURE 2, the bottom wall of the container is reinforcedby transversely extending channels 43. A drain valve 49 in provided forthe tank 30 for cleaning it out, as may be needed.

As shown in FIGURE 3, the tank is separated into zones, areas, orcompartments by vertically-extending separator plates 48 and, as shownin FIGURE 4, a group of electrodes comprising a pair of anode plates aand a cathode plate 0 is suspended within each compartment defined bythe separators 48. The electrode plates a and c are carried by hangers dwhich are suspended by their upper hook-like ends over the electrodebars 46 and 47 that are generally designated in FIGURES 5 and 6 as barsb. The lower ends of each hanger d are secured to the upper end of therespective electrode plates a and c. The anode plates a may be of asuitable material such as steel or stainless steel; and cathode plates cmay be of a suitable material such as stainless steel.

In FIGURES 5 and 6, I have shown details of the mounting of thelongitudinal-extending electrode bars b on the supporting channels 45. AU-shaped bolt or clamp 50 extends over the top edge of theedgewise-positioned electrode b, through a resin insulating piece orstrip 51, a resin or insulating bushing 52, and thrOugh a resin orinsulating washer 53 and a steel washer 54. It will be noted that thebushings 52 are positioned in holes or openings in the web of thechannel 45 to space and insulate legs of the clamps 50 with respectthereto. Each clamp or bolt 50 is maintained in a tight holding positionby nuts 50a which are secured on its threaded ends. It will thus be seenthat the mounting of each clamp or bolt 50 is insulated from the metalof the container and the frame by the insulation of the parts 51, 52,and 53.

As will be noted, the system of FIGURE 1 may be continuously operated,its circulating aqueous solution may be maintained at a requisite pH forgold recovery, and the amount of gold recovered during each passagethrough the treatment zone E may be based on the most economical basis,since any gold remaining in the solution is recirculated and not lost. Icustomarily adjust the DC. current up or down, as applied to theelectrodes at station E, to provide a power supply based on the goldcontent desired in the solution being returned or recirculated. Theoperation has been successful for recovery of gold from one or moreplating lines that employ one or more types of plating solutions. Inaddition, as found to be desirable, the pumps 26 and 32, providesuitable agitation of the solution during the plating out of gold on thecathode plates 0 of the tank 30.

Although for the purpose of illustrating my invention, I have shown anddescribed a representative system employing it and have shown anddescribed a suitable electrode recovery tank, it will be apparent tothose skilled in the art that various changes and modifications may bemade without departing from the spirit and scope thereof, as indicatedin the appended claims.

What I claim is:

1. An electroplating method of economically recovering gold in solutionfrom a washing water waste solution employed to wash oif dragout fromgold plated workpieces provided by a gold electroplating operation,including a waste solution having a low concentration of gold content,and irrespective of the type of solution used in the previous goldplating operation on the workpieces whether of a cyanide, acid orneutral type which comprises, collecting the waste solution from saidwashing operation, progressively converting the waste solution to analkaline cyanide solution having a pH above that at which poisonouscyanide gas will be evolved by adding alkali metal cyanide and hydroxideas required, progressively moving the converted solution through anelectroplating bath having anode and cathode plates, progressivelyplating out gold from the solution on the cathode plates in the form ofmetallic gold, and progressively returning said solution to said washingoperation.

2. An electroplating method of recovering gold as defined in claim 1wherein, the converted solution has a pH of about 12, and has a minimumcontent of about .5 oz./ gal. of cyanide in solution.

3. An electroplating method of recovering gold as defined in claim 1wherein the converted solution has a minimum pH of about 10.

4. An electroplating method of recovering gold as defined in claim 3wherein a minimum of about .5 02/ gal. of CN is maintained in theconverted solution.

5. An electroplating method as defined in claim 3 wherein electriccurrent supplied to the anode and cathode plates of the electroplatingbath is increased with a lower concentration of dissolved gold contentin the converted solution and is decreased for a greater concentrationof dissolved gold content therein.

6. A method as defined in claim 3 wherein the plated out gold is removedfrom the cathode plates as a peelable gold foil.

7. A method as defined in claim 3 wherein washing water is provided inbaths at the end of a group of electroplating lines along which goldplated workpieces are progressively passed which comprises,progressively washing off the gold plated workpieces in the baths at theend of the lines with water, progressively taking off the washing waterwaste solution from the baths of each of the plating lines andprogressively converting it to an alkaline cyanide solution having thedefined minimum pH, progressively moving the converted solution throughthe electroplating bath and progressively plating out gold from thesolution on the cathode plates thereof, and progressively removing thesolution from the electroplating bath and returning it to the baths ofthe lines of the group.

8. A method as defined in claim 3 wherein the solution is continuouslyagitated in the electroplating bath during the plating out of itssolution gold content.

9. An electroplating method of recovering gold as defined in claim 3wherein, a minimum of about .5 oz.. gal. of an alkali metal cyanide ismaintained in the converted solution, and controlled amounts of analkali metal hydroxide are added to the solution during its conversionto provide it with the defined minimum pH.

10. A method as defined in claim 9 wherein the alkali metal cyanide isof a class consisting of sodium and potassium cyanides and the alkalimetal hydroxide is of a class consisting of sodium and potassiumhydroxides.

11. A method as defined in claim 9 wherein, the alkali metal cyanide isinitially added to the waste solution and the alkali hydroxide isprogressively added to the waste solution in converting it for use inthe electroplating bath, and the solution in the electroplating bath isprogressively recycled for use as a dragout washing solution and forremoving its solution gold content.

12. A method as defined in claim 9 wherein the gold of the wastesolution is electroplated out on the cathode plates when its content isas low as about 5 mg./l.

13. An electroplating method of economically recovering gold in solutionfrom a washing water waste solution employed to wash off dra gout fromgold plated workpieces provided by a gold electroplating operation,including a waste solution having a low concentration of gold content,and irrespective of the type of solution used in the previous goldplating operation on the workpiece Whether of a cyanide, acid, orneutral type which comprises, collecting the waste solution,progressively converting the waste solution to an alkaline cyanidesolution having pH above about 10, maintaining a minimum of about .5oz./gal. of cyanide in said solution, progressively moving the convertedsolution through an electroplating bath having anode and cathode plates,and progressively plating out gold from the solution on the cathodeplates in the form of metallic gold.

Metal Finishing, July 1950, pp. 48-49; May 1968, p. 59; pp. 60-65.

ROBERT K. MIHALEK, Primary Examiner.

R. L. ANDREWS, Assistant Examiner.

U.S. Cl. X.R.

1. AN ELECTRPLATING METHOD OF ECONOMICALLY RECOVERING GOLD IN SOLUTIONFROM A WASHING WATER WASTE SOLUTION EMPLOYED TO WASH OFF DRAGOUT FROMGOLD PLATED WORKPIECES PROVIDED BY A GOLD ELECTROPLATING OPERATION,INCLUDING A WASTE SOLUTION HAVING A LOW CONCENTRATION OF GOLD CONTENT,AND IRRESPECTIVE OF THE TYPE OF SOLUTION USED IN THE PREVIOUS GOLDPLATING OPERATION ON THE WORKPIECES WHETHER OF A CYANIDE, ACID ORNEUTRAL TYPE WHICH COMPRISES, COLLECTING THE WASTE SOLUTION FROM SAIDWASHING OPERATION, PROGRESSIVELY CONVERTING THE WASTE SOLUTION TO ANALKALINE CYANIDE SOLUTION HAVING A PH ABOVE THAT AT WHICH POISONOUSCYANIDE GAS WILL BE EVOLVED BY ADDING ALKALI METAL CYANIDE AND HYDROXIDEAS REQUIRED, PROGRESSIVELY MOVING THE CONVERTED SOLUTION THROUGH ANELECTROPLATING BATH HAVING ANODE AND CATHODE PLATES. PROGRESSIVELYPLATING OUT GOLD FROM THE SOLUTION ON THE CATHOLE PLATES IN THE FORM OFMETALLIC GOLD, AND PROGRESSIVELY RETURNING SAID SOLUTION TO SAID WASHINGOPERATION.